1. Field of the Invention
The present invention relates to a method of producing an air electrode material for solid electrolyte type fuel cells.
2. Related Art Statement
Recently, solid electrolyte type fuel cells (usually called "SOFC") have attracted attention as exceedingly promising electric generators having advantages of a high generation efficiency, low public nuisance, and a capability of using diversity of fuels (naphtha, natural gas, methanol, reformed coal gas, etc.).
A current important task in developing SOFC is how to lower an overvoltage of the air electrode, which causes a voltage drop of SOFC.
Causes of over voltage include disturbance of electrode reaction and resistance due to an insulation layer formed on an intersurface between the air electrode and the stabilized zirconia solid electrolyte.
Syntheses of La.sub.0.6 Ca0.4MnO.sub.3 was reported, for example, in Mizusaki et al "Electrode Thickness, Microstructure and Electrode Properties of La.sub.0.6 Ca.sub.0.4 MO.sub.3 (M=Mn,Co)/YSZ System Air Electrode for High Temperature Solid Oxide Fuel Cells" described on Bulletin of the Chemical Society of Japan, [9], pp 1623-1629, 1988, wherein La.sub.2 O.sub.3, CaCO.sub.3 and Mn.sub.2 O.sub.3 powders are mixed in a ball mill with ethanol for 24 hours, once pulverized and sieved, again fired at 1,200.degree. C., and pulverized. The thus obtained synthetic La.sub.0.6 Ca.sub.0.4 MO.sub.3 powder is again pulverized in a mortar, mixed with terebinth oil to form a paste, applied on a surface of a solid electrolyte YSZ pellet, and baked at 1,100.degree. C. for 4 hours to prepare the material La.sub.0.6 Ca.sub.0.4 MnO.sub.3 of the air electrode. However, the method has a drawback in that cracks are formed in the air electrode due to a difference of thermal shrinkage between the solid electrolyte pellet and the air electrode if the application amount of the paste exceeds 50 mg/cm.sup.2. Also, reaction products are formed at the intersurface between the solid electrolyte pellet and the material La.sub.0.6 Ca.sub.0.4 MnO.sub.3 at the time of baking the fuel electrode onto the other surface of the solid electrolyte pellet at a high temperature (e.g., 1,300.degree. C.
On the other hand, instead of providing the coating of the air electrode on the surface of the porous support and sequentially arranging the solid electrolyte and the fuel electrode on the coating, a technique was proposed of using the porous air electrode as the sole structural support. By this technique, the whole structure of SOFC can be simplified as well as the production process, and the production cost can be reduced.
However, even in that technique, cracks are liable to form in the solid electrolyte or the air electrode at the time of coating the solid electrolyte on the surface of the porous air electrode at a high temperature of (e.g. 1,400.degree. C.) and the reaction products are formed at the interface between the solid electrolyte and the air electrode. Moreover, when the self-supporting type porous air electrode is used as the support, the porous air electrode is largely deformed and may shrink if it is exposed to such a high temperature as mentioned above.